How fast should a hot laptop's fan spin?

I am having some problems with my T420 Lenovo laptop overheating. The fan is working, in that I feel air being blown and can hear it when I put my ear near the heat sink fins. I have noticed however, that I no longer hear the fan speed increase noticeably, even when the laptop is under load.

The laptop is running a fully patched OpenSUSE 13.2 Linux distribution.

This is the output of sensors when the system is not under load:

acpitz-virtual-0 Adapter: Virtual device  
temp1: +53.0°C  (crit = +98.0°C)

thinkpad-isa-0000 Adapter: ISA adapter fan1:  
     3575 RPM

coretemp-isa-0000 Adapter: ISA adapter  
Physical id 0:  +53.0°C  (high = +86.0°C, crit = +100.0°C)  
Core 0:         +53.0°C  (high = +86.0°C, crit = +100.0°C)  
Core 1:         +48.0°C  (high = +86.0°C, crit = +100.0°C)

And output when under load:

acpitz-virtual-0 Adapter: Virtual device  
temp1:        +90.0°C  (crit = +98.0°C)

thinkpad-isa-0000 Adapter: ISA adapter fan1:  
     3868 RPM

coretemp-isa-0000 Adapter: ISA adapter  
Physical id 0:  +93.0°C  (high = +86.0°C, crit = +100.0°C)  
Core 0:         +92.0°C  (high = +86.0°C, crit = +100.0°C)  
Core 1:         +93.0°C  (high = +86.0°C, crit = +100.0°C)

According to sensors, the fan speed increases ~8% when nearing critical temperatures.

I think the fan is set to auto speed.

cat /proc/acpi/ibm/fan 
status:         enabled
speed:          3591
level:          auto

How fast should the fan be spinning when under load? I think it should be faster because I can remember the fan noise being easily audible at times months ago.

And if there is a problem, how can I troubleshoot the OS fan control?

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