SUV has problem getting up to 30 40 mph

Hard to get vehicle to go, can be turned off and restarted to go for a bit, then struggle to go.
Took to nearby rpm shop, they say it's the transmission.
I drove recently over 50 miles around town and on freeway, no signs of transmission problems.
I had front end alignment done maybe 1,500 to 2,000 miles ago, the mechanic drove it around to check on front end, no problems in transmission then were noticed.

I suspect the catalytic converter.
Rpm says that's not the problem.

My grandson drives it daily on interstate highway 60 miles round trip to school.
Was driving it.