Riot doesn't think the tower changes in 7.23 had any effect on game length or first towers falling—more changes to come
Patch 7.23 introduced a few changes to turrets in League of Legends. They were given more health, minions now deal more damage to them, and minions spawn faster after 20 minutes.
These changes were pushed to help the pace of the game. More specifically, they were pushed to slow things down a bit. Both the game duration and timing of the first tower falling were meant to be less rushed. This was designed to help splitpush champions a bit, too. If they are able to manipulate the minion wave correctly, they should be able to use their amplified damage to take towers faster when left alone in lane.
Unfortunately, almost none of that actually succeeded, according to Riot's post on the game forums today.
The average time it takes teams to bring down the first tower has been extended by six short seconds, and that's it. Otherwise, there has been nearly no changes to the game's pace nor have splitpushers become more viable.
On that note, Riot feels that the original approach may not have been enough, which means more changes may be on the horizon for turrets, minions, and maybe even splitpush champions that have been feeling weaker than usual.
It's possible that the changes did work at least a little bit, but something else happened to negate it. For example, Riot stated that more support players are angling towards the Frost Queen's Claim item line, which means there are more damage dealers in the bot lane and less peel and crowd control to protect high priority targets. This means that there's more fighting and more dying, which may speed up the pace of the game and overlap with the changes made to slow things down.
The next patch, Patch 7.24B, will be strictly focused on game balance, which means there may be more changes introduced to ensure that they are live before the season officially begins on Jan. 9.