If you recall from TNG 'Descent' episodes (parts I and II), the Enterprise-D encountered Transwarp conduits which were apparently set up by the Borg.
Since those conduits were set up decades ago (and by renegade Borg separated from the Collective), its very possible those conduits operate on slightly different principles and as a result are slower than the ones we saw the Borg use later on in the DQ.
After all, technology advances (exponentially) and the renegade Borg in TNG would have only had the working knowledge to create TW conduits as they were possible back then.
From Star Trek TNG Descent Part I:
PICARD: How fast would a ship travel through one of these conduits?
LAFORGE: We don't know. Normal subspace limitations don't apply to
transwarp variables. But I'd say based on the distance we covered
during our trip through the conduit, the speed would have to be at
least twenty times faster than our maximum warp.
In Star Trek Voyager however it was established by Tom Paris in episode 'The 37's' that Warp 9.9 = 4 billion miles per second (which equates to 21 473 times speed of light).
That would mean that the TW conduits from ST: Picard are actually 40.8 times faster than Warp 9.9 (but then again, Warp speed was also A LOT faster in early TNG - remember that in TNG Season 1, it would have taken Ent-D approximately just over 300 years to traverse 2.7 million Ly's... indicating a speed of 9000 times speed of light (at a theoretical maximum warp speed of 9.2-9.6 at the time of TNG Season 1).
It wasn't until Voyager came along the writers muddied the numbers by quite a bit, intentionally dropping Warp speeds to really low levels to make the galaxy seem 'vast' (which if you want my personal opinion was completely unnecessary and wrong - they could have just thrown Voyager well beyond the galaxy - millions of ly's away and said it would take the ship 75 years to get back at maximum sustainable warp).
Also, TNG Enterprise-D was usually only able to reach and maintain Warp 9.6 as an upper stable/sustained speed, however, I suppose its 'theoretical maximum' might have been 9.9 for a very limited time frame (and Geordi DID say 'at least'... so that leaves considerable margin for difference).
Another quote from ST: Descent Part I:
PICARD: Report.
RIKER: Navigational sensors show that we've travelled
sixty five light years from our previous position.
65 Ly's from a relatively brief jolt of 8-10 seconds if it was supposed to be real time... however, we were never told how long the ship spent inside the conduit.
If it was indeed 65 Ly's in about 10 seconds, that would imply a speed of conservative 390 Lightyears per minute, and 23,400 Lightyears in just 1 hour (or 561,600 LY's per day which translates to 204,984,000 times speed of light).
Voyager was quoted on a few occasions that its 'maximum sustainable cruise velocity' is Warp 9.975.
Given that Warp speeds increase in velocity exponentially past every increment after Warp 9.9, Warp 9.975 would actually be equivalent to roughly 400 Ly's per hour... or basically faster than Quantum Slipstream version 1 (which allowed Voyager to traverse 300 Ly's in 1 hour) and those Borg TW conduits from ST: Picard.
Incidentally, Quantum Slipstream version 2 as seen in ST VOY: Timeless has a speed of approximately 10 000 Ly's per 1 minute (which is even faster than those Borg TW conduits from TNG Descent).
And similarly, The Borg Transwarp Hub seemed to have produced similarly fast TW conduits (which allowed Voyager to get back to Federation space).
Since Voyager's ride inside the Hub's TW conduits was obviously a few minutes long, we have to assume that those TW conduits were slower than QS V2 by roughly half as much (why? mainly because by that point, Voyager was roughly 16 000 ly's away from Earth - remember that at some point in Season 7, Q2 episode had Q doing Voy a favour and bringing them a bit closer to Earth - it wasn't stated just how much closer, but its estimated Voyager was before that 30 000 Ly's away from Earth... and Q brought them closer by about 14 000 Ly's).
Since we never saw Voyager achieve or sustain Warp 9.975 (remember that in the episode 'Threshold', the ship started shaking itself apart when reaching 9.9 with the computer warning they are reaching 'structural collapse, Chakotay was forced to drop speed to Warp 9.5), we have to reach a conclusion that Voyager's engines were either damaged in their transition to the Delta Quadrant which the crew was not able to repair, preventing them from achieving and maintain their 'maximum sustainable cruise velocity', or the vessel was never capable of reaching Warp 9.975 in the first place and it was an 'error', and in fact were only capable of reaching 9.75 for a limited period as evidenced by episode 'The Swarm' :
TUVOK: They appear not to have detected us. They have not powered up
engines.
JANEWAY: All right, let's get through this as fast as we can.
Mister Paris, what's your recommendation?
PARIS: I'll try holding warp nine point seven five for as long as I can.
If Warp 9.975 was a 'maximum sustainable cruise velocity' for Voyager, that means the ship would have been able to reach and maintain it indefinitely (or for as long as they have the fuel)... and it would have returned home in less than 7 days.
And while this Warp factor (9.975) seems 'consistent' with an exponential jump in technology and science since TNG (which fits for what the Federation in 24th century should be able to achieve in the 7-10 years time frame), the overall series TNG, DS9 and VOY made it clear that no SF ship on a consistent basis was able to go past Warp 9.9 (except under extreme conditions which involved alien influence or anomalies - and those were rather infrequent).
In fact, most Starfleet ships were achieving speeds BELOW 9.9 (it wasn't until the USS Prometheus was launched (Voyager Season 4 - Message in a Bottle) that that particular ship was able to seemingly achieve/sustain Warp of 9.9 without effort - it was stated by VOY EMH that the Prometheus was travelling at Warp 9.9 straight for Romulan space).
Anyway... the TW conduits as seen in ST:Voyager were shown to be consistently faster (than the ones in ST: TNG Descent or Picard) and this could be attributed to technological progression made by the Borg since TNG, and we've seen them either generating their own TW conduits through use of TW coils, or that the conduits can be set up by use of 'subspace manifolds' which keep the conduits intact/operational (though the manifolds were seen in use for the Borg TW network which spanned the Galaxy and was subsequently 'obliterated' as stated by 7 of 9 at the end of Voyager series).
So, the TW conduits seen in ST: Picard could either be the same ones we saw in TNG Descent (just never used/explored), but the reason for their seemingly much lower speed could be multi-factored... lack of subspace manifolds, they were generated differently or they degraded from lack of maintenance/use/whatever (this last one could easily be the case).
They are still (apparently) faster than your highest possible sustainable Warp as seen in TNG/DS9/VOY... however, we also don't know how much faster SF ships have gotten in ST: Picard, nor do we know how fast is the La Sirena (but it seems the writers apparently kept the element of 'wow' for TW conduits being faster than Warp - which is odd because you'd think by this point the Federation would be zipping around the Milky Way galaxy in minutes from everything they learned in previous series ranging from TNG, DS9 and VOY (especially when you think that science and technology evolve exponentially) and these speeds from TW conduits would be a 'roll eye' moment - if anything, they would prompt a 'why so slow?' question - but it WAS a minor clip to be honest and we won't know if they have more to say on this subject in the upcoming episode.
