Vostro 620, CPU fan speed slow, high temperature

no conditions stated (cpu load nor room temps)  Do you have and ambient case thermal sensors?

if the ambient sensor in the case is too hot then how can that CPU fan work well,  it can't or limited.

50c is not  hot,, 90c is.

ever seen fan profiles on top mobo makers,? 

you can even set the fans to not turn until temps hit 50c, (where ever you like) but that is global answer. not dells.

or run linear curve from (from min to max , 30c to 90C)

the fans also have minimum allowed RPM and that is where it starts. some go to 300 rpm slow.

I have no idea what dell allows here, just telling your choices, if you want better MOBO buy one. !

vastly better motherboards exist, for sure this topic heat and noise, I hear an echo ,Asus. calling,

The fan is passive cooling.

AIDA64 and other 3rd party software is not supported by Dell.

There is no guarantee that what is showing is accurate.

I would try the intel power gadget and see if the cpu is throttling.

Monitoring Processor Energy Usage with Intel® Power Gadget 2.0

Download Intel® Power Gadget 2.0 now!
32-bit Intel® Power Gadget 2.0 (Windows)
64-bit Intel® Power Gadget 2.0 (Windows)

Intel® Power Gadget 2.0 for Mac

Windows 7* and 32-bit versions of the Intel® Power Gadget for Windows* has ceased development from 3.0.7. Starting with version 3.5 and going forward, only the 64-bit version and Windows 8* will be supported.

Version 3.5 is the latest but only for 64 bit windows

https://software.intel.com/en-us/articles/intel-power-gadget/

Intel® Power Gadget is a software-based power usage monitoring tool enabled for Intel® Core™ processors (from 2nd Generation up to 10th Generation Intel® Core™ processors). Intel® Atom™ processors are not supported. It is supported on Windows* and Mac OS X* and includes an application, driver, and libraries to monitor and estimate real-time processor package power information in watts using the energy counters in the processor. With this release, we are providing functionality to evaluate power information on various platforms including notebooks, desktops and servers.

op by passive cooling that means in simple words

the fan  can be off in a PC , until the BIOS trips and fan comes on , called dynamic cooling.

Passive means metal heat sink only cooling , metal to air.

the CPU can also self cool via 2 ways, it slows down, if not used, (programmable by you) and if it overheats

it also slows down, to produce  less heat and cool;.

The BIOS runs the fan,  ok. The BIOS has fan tables with setting for all temperatures.

what you can change there IDK,  but Dell PCs as class, very little unless "Alienware"

There are many programs that let you monitor temps, and many fail. try HWinfo32.exe. it's free and safe.

sure intel software runs on intel processors. sure. (processor data sheet even teaches you how)