Immigration lines can be slow at both KLIA and KLIA 2, although I doubt I ever spent longer than 20 - 30 minutes in either. I fully recommend doing the day trip, as mentioned up thread, you have enough time to do quite a "few" things. KILA Express will drop you at KL Sentral Station. From there you can take a Grab car to get a shot of the Petronas Towers on the corner of Jalan Ampang and Jalan Ramlee. You can walk from there over to the KLCC shopping mall for a bit of lunch (food court is decent), or just a bit of wandering. After a that, you can take another grab over to the Batu Caves. I wouldn't recommend going up the steps, not that fascinating up there, but still a photo outside is probably warranted. After, you can take Grab again, this time over to Bukit Bintang. Quite touristy on that end, but worth a stop. If you want local fare, it will be a bit early for the closest true hawker experience (Jalan Alor), but you can visit the food court in the basement of the LOT 10 shopping plaza. They have a great selection of dishes from across Malaysia, really good I'd say. After eating again, you can head over to the Pavilion shopping plaza for a bit of shopping or wandering in AirCon. If you want to try KL tower and still have time, consider taking the ride up.

Not a whole lot to do in KL other than eat from my perspective. Lots of great restaurants and bars, but as you will not stay the night, not worth mentioning. I was on assignment for 6 months in 2016/2017 and generally left the city for trips to other countries in true FlyerTalker fashion. Visited all of ASEAN minus the Philippines . I go back 2 to 3 times per year.

In short, you have more than enough time to get a flavor for KL. Note, traffic can be terrible in the city center and the KLIA Express is the fastest way to get into KL. Don't bother with a taxi until you are in town. If you are comfortable with taking public transportation, LRT / Monorail are okay options as well. Use Grab, regular taxis will generally be a hassle.

Cheers