First, some facts :-

jD and Java won't have any effect on your download speed. Have a look at the code. ;)

Speed in jD can be affected by a wrong configuration of your firewall or virus scanner. Most firewalls cause problems handling java programs, because some java tools are using the same .exe and there is no way for the firewall to tell the difference between jD and some other java tools. Did you allow java(.exe), javaw(.exe) and JDownloader in your firewall? Disable http/web scanning in your anti-virus software. Known software that cause such problems are Antivir, Avast!, AVG, Eset, Nod32, Kaspersky, and Zonealarm among others.

Never trust browsers when doing speed tests. jD calculates the average download speed better than Firefox. For instance, RS sends data, pauses, sends data, pauses and so on. This causes many tools to report wrong speeds. Firefox is not able to calculate average speed for RS free customers very accurately because of the way RS caps speed. You can read about it in our forum, in the gulli forum...everywhere it's written Firefox lies about the real speed. (source: Jiaz)

Speed depends on factors like host server (mirror) speed, overall network traffic, ISP, behaviour of other Internet tools on your PC.

A speed test with one file depends on jD's configuration. Set Max.Con.=1, Max.Dls=1, Speed=0

EDIT: Some people experience burst bandwidth (I'm not sure this is the best term) when a site or ISP are sending data at an unusually high rate compared to their normal rates. This burst will often stop after a few seconds or megabytes.

RS caps the download speed for free customers to max 125kb/s, but it varies with server load. Premium accounts should deliver higher speeds. That's why you pay for them. ;)


Second, some measuring tools :-

There are differences in the quality of speed metering in different software. You should use a good stop watch or dedicated traffic metering software in order to measure the real speeds.

A simple approach is the Task Manager and its Netgraph while downloading the same file from the same server in Firefox and jD. You could take a screen shot which is a simple snapshot in time.

Simple Internet Meter ("**External links are only visible to Support Staff**) will show accurate values for download speed and it has a 14 day trial period.
Cfos ("**External links are only visible to Support Staff**) might seem a little complicated for beginners so it's best to try the simple internet meter first.
(source: Freeloader)


Third, real speed tests :-

Real jD speed tests can only be done with a dedicated root server. Loading files from RS or any other file host will always show different results with every test run, because of a different load or server selection on the host side. By using a dedicated server and file you know that the line is free and you are the only one downloading the same file. (source: Jiaz)


Fourth, statistics :-

All the discussions about speeds and throughput are useless without a correct, statistical measuring system that computes the true downloading performance over time.

Good statistics could level differences out in a period of a day or a week. A neutral and freeware tool like MZL & Novatech TrafficStatistic for Windoze ("**External links are only visible to Support Staff**) might do the job.

How to report a speed problem