Supermarine Spitfire. At Royal Air Force's service. Part nine. The early Mk. IX.

 

In order to cope with the dusty north African climate, the Spitfires were fitted with a large Vokes air filter under the nose, which was detrimental to Spitfire's performance due to the increased drag, this sub-variant, called Mk. Vb (trop) or Mk.Vc (trop), depending on the type of wing, equipped many squadrons of Royal Air Force's (RAF) Desert Air Force during the North African Campaign during the summer of 1942.
On this operational theatre, the Mk.Vc was also employed in the tactical fighter-bomber role, carrying a maximum payload of 500 lb of bombs (227 kg).
One of the main antagonist of the Spitfire on Malta and North Africa was the Italian Macchi C.202 fighter. This fighter was often regarded as superior to both the Hawker Hurricane and the Curtiss P-40, which were, until the Spitfire was introduced, the main Allied fighters in North Africa, as the C.202 was, in terms of performance, a close equal to the Spitfire. 
The Spitfire met the C.202 for the first time on Libya in November 1941 and it was claimed the C.202 could outrun all three types of fighters, although the Spitfire was better armed and had a superior rate of climb. 
The same could be said about the other three main Italian fighters, the Reggiane Re.2001, Reggiane Re.2005 and the Macchi C.205, they had good performance, comparable to that of the Spitfire and the German Messerschmitt Bf.109, but  they lacked heavy armament, which hampered them. 

Moving on to the European Theatre of Operations (ETO), when the German Focke-Wulf Fw.190 was introduced in late 1941 it came as an absolute shock for the RAF. Initially it was thought that they were Curtiss 75 fighters captured from the French, due to their radial engines, however it was soon clear that the new fighter could easily outperform the Spitfire Mk. V and was also much better armed than the Curtiss. In fact, the Fw.190 was a mystery for the RAF until 23rd June 1942 when one Fw.190A-3 belonging to Jagdgeschwader 2 landed at RAF Pembrey, Carmarthenshire, Wales, by mistake. After performing some comparison flights against British fighters, the Fw.190 proved superior to every aspect, except turning radius. 
Given the high number of casualties that was being inflicted on Spitfires, the Focke-Wulf was dominating the skies in the ETO for the most part of 1942. It was during the later part of that year that, in order to achieve some degree of parity, that the squadrons received the Spitfire LF Mk.Vb. This variant featured reduced diameter supercharger impeller blades on the Merlin engine for optimum performance at lower altitudes and the wing tips were removed and replaced by short fairings to improve their roll-rate. 
Meanwhile, on parallel, the Mk. IX was being developed and in April 1942 a modified Mk. Vc, coded AB505 was tested by the Air Fighting Development Unit (AFDU). This variant was powered, at first, by the much improved Merlin 61 engine, which could deliver up to 1,720 hp of power and was armed with two 20mm Hispano Mk. II cannons and four .303in Browning machine guns, in the "C" wing configuration. 
The Merlin engine had also various configurations depending on the altitude, so the Merlin 70 was optimized for high altitudes, the Merlin 63 for medium altitudes and the Merlin 66 for low altitudes. This led to the use of prefixes, applied to various marks, VII through IX, depending on which engine was installed. For instance a Mk. IX with the Merlin 63 and the "C" type of wing, would be officially marked as "Spitfire F.Mk.IXc"
In July 1942 the flight performance of the early Mk. IX was found comparable to that of the Fw.190, at any altitude, although the AFDU noted that the Fw.190 was "a little faster". Although the Mk. IX had better turning radius, it could be outdived and outrolled by the Fw. 190. 

Sources:
1st https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_(late_Merlin-powered_variants)