Three point shooting and turnovers are the main things people see I think.
Turnovers went from 3.6 to 4.1 a game while his assists went from 4.7 to 6.8 a game. He's averaging half a turnover more this year but he's also averaging 2 more assists, that's a fair trade, IMO.
His 3pt shooting went from 38% to 32% this year but his overall FG% went up from 43.2% to 45.4%. He's attempting just 6.5 3's a game this year to 7.1 last year as well, so I'll give him credit for lowering his attempts some while his shooting from out there has dipped. He's not getting nearly as many catch and shoot opportunities this year as he's been the primary PG for 2/3 of his time on the court.
His FG% on 2pt attempts has gone up substantially, which tells me he's getting to the basket for easier shots more often. Last year he shot 46% to this year's 52.1%. His FT attempts have gone up from 6.4 a game to 7.1 a game, which backs up the idea he's getting better looks closer to the basket. He shot 88% from the line last year and 86% this year, which is very good overall and is not enough of a change either way to worry about it.
Booker's eFG% went up from 50.1% to 50.7%, which isn't much but it is an improvement, and his TS% went up from 56.1% to 57.1%. He's doing all that while scoring more, 25.6 ppg compared to 24.9 ppg, and with 9 new teammates, 3 of which are starters with him, and a new coach.
So by almost any and every measurable stat that gets brought up about a players efficiency I would say Booker has become more efficient if anything, he hasn't regressed though. I could get behind the idea he hasn't improved as much as we'd like but we've seen him engaged on defense since the All-Star break with Oubre and I'll take that as an improvement this year. That won't show up in stats because it was only a dozen or so games but we should look for that version of Booker next year, at the least.
Source for all stats, Basketball-Reference