I wouldn't say they take up more time than an other character. A pure hacker though can, as Hacking can get rather intense and there are a lot of 'wait while I handle this moment, guys'.
4th Edition Shadowrun has really pushed a reason for the 'on site' Hacker and Mage so that can keep everyone together for the action.
Technomancers, especially early character ones, tend to be best suited as remote hackers (in VR) or Riggers (which can have a solid presence, though they are still in VR usually) and as such require a lot more of a specialization.
So my warning isn't really against Techomancers, but 'a high hacking focus' character can both be time intense for 1 PC and also a bit more rules heavy for a new GM/Group. And unlike a Hacker, Technomancers have a bit more trouble branching out into doing something besides Hacking (save maybe being the Face)
Threading (and overthreading) Allows a technomancer to 'know' a Complex form (program) they don't already know...or use it at a higher level. Overthreading lets them go above their Resonance (up to double). So a starting Technomancer with a Resonance of 5 (which is a likely starting Res) can thread say...the Stealth program upto 10. (they have to resist 'fade' which is the like Drain for mages) and it would be Physical damage, but having a stealth 10 is VERY nice as a starting character.
The Technomancer side of things play very much like a Magician does. Threading is 'casting and sustaining' a spell and Spirtes are just like Spirits (more or less. there are a few minor details, but honestly, it was intentionally done so Magic and Technomancer aren't weird different systems).
And it's not that a Technomancer can't also shoot a gun or be sneaky or whatever, but to really do their Hacking job, Technomancers need to really invest into their Technomancer abilities, which leaves them less 'spread around' points and cyberware costs them abilities as well, so the 'cheap and easy' boosts from 'Ware are less attractive for them as well.
So my caution is more 'just make sure you and the player(s) know what to expect from campaign tone and style.' If Hacking is going to be a big part of the game, then a Technomancer will shine. If it's something you want to downplay to avoid the bog down of rules and the like, then a technomancer might not be the best choice. But that's really the same for any concept. You want to make sure the concepts of the PC's match the tone/style of the story. A high spy intrigue game would make an Uncouth pure combat Sam likely to be very bored and maybe even a detriment to the group.