Is replacing a hard drive for an SSD always worth it?

disc transfer speed don't depend on memory amount or cpu speed

HD-to-SSD change has other effects. HDs are spinning platters with a read/write head seeking much of the time. SSDs have faster seek times because of direct addressing. Also the "swap file" that used to be moved to/from in addition to the R/W-head seeks is changed to direct-access R/W of an SSD.

Startup should be faster, and opening applications should be faster.

Something seems wrong.

What SSD did you install?

What does system profiler shows about the dis and the disk control?

Go to SATA/SATA Express in profiler

OS X: About System Information and System Profiler - Apple Support

and post a screens shot when the controller is selected

How to take a screenshot on your Mac - Apple Support

A current third-party SATA 3 SSD should be showing over 500MB/s read and write performance. As has been noted, something is not correct with your getting the results that you posted. Depending on the brand, it may require a firmware update for improved performance, and/or OS X compatibility.

This doesn't really solve your problem, but as a reference, I have a Kingston v300

SSDNow drive installed in my 2011 Mini Server. I get reads ranging from 300-450

megabytes/second writes ranging from 180-250 megabytes/second. One thing, these

are not the speediest SSDs on the market, but are faster than an HDD.

Something to note, running the speed test on an active boot volume (depending on the app)

can give wildly variable results as it will typically be competing with other system accesses.

I ran the test multiple tomes to get the range that I reported.

If possible, you may want to create a clone to an external drive, boot to it, and then redo the test.

The link speed is 6 GB/sec which is correct..

You either have a bad drive or as woodmiester said running the test on a boot drive may be the cause of the low speeds.

As already mentioned something is wrong. You should try to update the firmware if a newer one is available. If you cannot get it faster you should return it and/or get a warranty replacement.

BTW, you should enable TRIM. In Terminal, issue the command "sudo trimforce enable". You will have to enter your password. Afterwards TRIM should be enabled also on 3rd-party SSD drives, which you should see in System Profiler as "TRIM Support: Yes".