MechaTarrasque
Hero
Summoning has always done this. It is a well-documented way to bog down the game and break the system. All 5E did over 3X(the system they modeled the majority of 5E after) was make it take 30 minutes instead of 45. And frankly, I don't think it makes a lick of difference is Bob takes one half-hour turn, or six 5-minute turns. The end result is still that it takes Bob considerably more time, slows down the game and is more likely to break the encounter.
If they wanted to actually fix summoning (which they didn't) then the solution is simple: summons work like concentration spells and you lose your action to command them instead. Multiple summoned creatures would act like swarms, attacking the same target and sharing the damage (to keep things quick and simple), single summons would work as "replacement" characters. While Bill the summoner runs around trying not to get hit, his Mammoth whoops booty.
But 5E really didn't want to address these issues. And their half-hearted attempts left the wound smaller, but still open.
It's one of the reasons I've largely gone back to 3X, if I've going to play a system with all the same flaws, I'm going to play the one with vastly more content.
4e summoning tended to function like that (with the pleasant [for DM's] aspect that if you summoned a demon and didn't use your action to have it attack something, you suffered some consequence [particularly bad if you wasted a balor's time]). It wasn't perfect (the summoned critters didn't have all the abilities of the nonsummoned critters, I.e., why can't my pit fiend do the stuff that other pit fiends do?), but I think the devs might have thrown the baby out with the bathwater in trying for a more pre-4e style of summoning. It reduced issues of questions about what you get when you summoned something (each spell was for a specific type of demon/elemental/angel (for invokers), ditto for class abilities for the warlock's allies (fey/elementals/abberations/devils)* and kept the battlefield clutter down (no use summoning a bunch of minions if you could only control one)
As for mass summoning, I suspect that was put in to support mass combat that never really took off. Those type of spells could have waited for a mass combat module to come out.
* They tried to bring this back in the Old Black Magic UA (where spells summoned specific types of demons), but it must not have playtested well. I will say the way 5e conjure spells pick monsters does open up an ever increasing list of options, which I have to admit beats the old "here is your list of monsters, which you can expand if you take this feat or get that boon."