D&D 5E Is Tasha's slowing down your combat?

[Dausuul]

I also cannot comment from personal experience; I'd just like to point out that the OP contains a pertinent question that the designers have worried about the last few years. Pets (or more specifically summoned/conjured aids to the PCs) caused quite a bit of a stir in 3e, as combined with ubiquitous layering of spell effects, could dominate encounters and push fellow PCs out of the limelight in the right circumstances. Since then, the devs have been vary cautious with summoning in 4e & 5e, much to the disappointment of summoning fans. The whole sacrifice an action to let your pet have an action was a direct carryover from 4e and seen as a way of limiting pets from dominating the action and taking too much time.

The funny thing is that "sacrifice an action to let your pet act" makes perfect sense for summoning spells--controlling the summoned creature takes your full attention--and that's the one place where it doesn't normally show up in 5E. It would make it possible to power-up summons significantly, since the summon is replacing a full PC instead of just a single spell slot.

 

[Stoutstien]

I can see combat slowing down but not due to the new summon X spells.

There is a lot more dice manipulation options across the board for players so that can potentially slow the game down.