I can't speak for Mexicali/Calexico specifically, as even though I live less than 3 hours drive away I've never been there. However, I can share my experience and observations having crossed the border by foot many times at the two main Ports of Entry between Tijuana and San Diego.
At both San Ysidro and Otay Mesa Ports of Entry there are two lanes for pedestrians - one General lane for all travelers, the second lane exclusively for SENTRI cardholders (which includes Global Entry and NEXUS cardholders). US Citizens without one of these cards cannot use the SENTRI lane.
The busiest/slowest times for crossing from Tijuana into San Diego is in the mornings, as most of the traffic (car and by foot) is morning commute. The reverse is true in the evenings. So for someone doing a day trip into Mexico, you'd be travelling the opposite direction, so would normally be fairly quick, even in the General lane.
I am guessing that Mexicali/Calexico may not have the same commute based times as Tijuana/San Diego, simply due to Calexico not being the business mecca.
With SENTRI I have never experienced a wait while entering the US on foot. The longest wait I've had in the General lane was 3 hours, in the morning commute.
The CBP has a Border Wait Times website which shows the current and normal average wait times for each port of entry. Below are links to the two pedestrian ports from Mexicali.
(I do see the website lists General and Ready - The Ready Lanes for vehicles is for people with Passport Cards, however I have asked a friend who travels regularly with Passport Card and he tells me that he cannot use it in the extra lane at the border, only people with SENTRI/Global/NEXUS. The signs above the pedestrian lanes at the border specifically say "SENTRI ONLY". This may be different at Mexicali)
My wife took this photo yesterday at San Ysidro, showing the General and SENTRI Only pedestrian lanes.