There is this:
A comparison of electric vehicle Level 1 and Level 2 charging efficiency
Also, on my previous 2017 Prius Prime, level 1 was around 80% efficiency versus 95% for level 2.
EDIT: Oh sorry, just noticed you were talking about capacity losses from charging more slowly being better.
My experience as well. Level 2 is a lot more efficient than Level 1.
Also, consider other issues. I just got our Escape PHEV a few weeks ago so haven't deep dived into its charging system yet. But I have had our Pacifica under the microscope for 4 years. One problem that became apparent is that the 12v battery system on the Pacifica is charged whenever the vehicle is ON and whenever the HV battery is being charged. This means that if you operate your car normally and drive it a couple hours a day on average, then charge it using L1 for 12+ hours, the 12v battery is being charged for 14+ hours per day. Normally I would say this is a lot of charge time for a typical 12v lead acid battery, but not necessarily a problem in a "normal" car. However, add to this a few other conditions unique to this PHEV and you have a potential problem.
1. PHEVs do not have a starter that traditionally lets an operator know when the 12v battery is degrading.
2. The Pacifica PHEV's HV battery systems "bootstrap" off the 12v system. That is to say, unless the 12v battery holds voltage above 10v during startup loading of the HV system when you push the start button, the HV system will not start or will have problems. Typically the car just acts completely dead when this happens and will not start up. You can have an HV battery with plenty of charge left in it but no way to boot up so you are dead until you get a jump.
3. The Pacifica PHEV 12v charging system operates whenever the HV battery system is active and applies voltage of 14.1v to 14.7v to the 12v system. It typically runs around 14.5v to 14.6v even though the factory supplied battery is placarded in red print not to exceed 14.4v charge voltage! This voltage limit is typical of 12v AGM batteries used in cars.
So why do I mention this? Because it is the reason many Pacifica owners are surprised one day to find their car completely dead with no warning. As long as the 12v battery can stay above 10v during HV boot, everything appears normal. This happens because there is no monitoring system that tracks the 12v battery degradation and warns the operator to get a new one. Then one day, ''surprise!" Also, If I use the factory L1 EVSE, instead of an L2, the charge time on my Pacifica's 12v battery (being overcharged at that!) is going to be about 13 to 14 hours per day instead of 5 to 6 hours. Compare this charge time to a "normal" car that charges its 12v battery about 1 to 2 hours per day on average. BIG DIFFERENCE, especially considering it is being overvoltaged by the design of the Pacifica's 12v charging system during this time. This wouldn't normally bother me if the voltage was controlled to float level of 13.5v to 13.8v once 100% charge capacity was reached on the 12v battery, but no such regulation exists.
Because of the frequency with which dead 12v batteries were occurring when I bought my Pacifica PHEV in 2018, I equipped it with a monitoring system that I have been gathering data from for the past 4 years.
Bottom line: The Pacifica PHEV has a 12v battery charging problem where, by design, it overcharges its 12v battery (aggravated by using L1 charging) and shortens 12v battery life while not providing a warning to the operator that 12v battery EOL is near.
After a few tests on the Escape PHEV and some research, I will decide whether I will instrument it or not. (Anybody know the HV boot sequence and source in the Escape PHEV?)
BTW: IF I assume a charge efficiency of 80% and 10.5 hrs during L1 and 95% and 3.5 hrs during L2 over 100k miles of electric-only (EV) operation, my current cost of $0.11/kWhr, 14.4 kWhr/charge, and 40 miles average on a charge, THEN:
L1 charging will take about 26,250 hours and cost about $4950
L2 charging will take about 8,750 hours and cost about $4168
So I can spend $782 on an L2 charger, break even on cost, and save myself 17,500 hours of charge time.
Seems worth it.
Besides, I expect my cost of electricity to rise and that I can sell my new L1 charger, which both make an L2 charger more attractive and cost effective.