Attempting to cast Photon Blast Barrage for any large amount of mana with lots of targets is an abysmal experience. Not only does it take forever to actually cast it and try and figure out what copies of the spell go where, it also doesn't let you figure out what you've already done, because it doesn't actually target until it resolves, so you don't know what you're casting it on.

If you have an opposing board where there's a bunch of 3/2s, 2/1s and so on, it's extremely easy to “miss” what you've targeted because the MTGA UI for spells like this is done so badly. You can't really hover over targets to see if PBB has targeted something because the way they stack at the edge of the screen either covers up targets or doesn't show you what copy of the spell is targeting what creature, because the "floating arrow pointing method" is too ambiguous when there's too many copies.

Lastly, the spell resolving takes so long that, for some reason, the game considers this part of your timer and times you out while waiting for the spell to resolve. It isn't like you have anything to do with this.

I didn't take a SS but really, just try casting PBB for X=12 or something and see what happens. In real life, this is manageable, but on MTGA, nearly impossible.

Suggestion:
Have cards with X=5 or greater targets float the "target" effects above the targeted creatures instead of crammed into a giant stack on the side of the screen. Let you arrange them by dragging them around instead of having to back out of the entire spell if you think you've missed something, which takes forever and you will time out.